As a structure suitable for facilitating a highly efficient operation of a high-frequency power amplifier, a system in which a plurality of output stage amplifiers are prepared and the respective output stage amplifiers are used so as to be switched by a switch according to an output level is disclosed in Japanese Patent Laid-Open No. H07-336168. That is, there is disclosed a power amplifier in which a first amplifying circuit for performing amplification of a high power level by a highly efficient operation, and a second amplifying circuit for performing amplification output of a low power level by a highly efficient operation are connected in parallel, and an FET switch is inserted in the output side of the second amplifying circuit.
On the other hand, as a way to improve efficiency, a structure in which an amplifier is used so as to be switched without a switch for an output stage amplifier is proposed in Japanese Patent Laid-Open No. 2003-046340, Japanese Patent Laid-Open No. 2004-134823 or in “School of Engineering Information and Communications University (ICU), A power Efficient W-CDMA Smart Power Amplifier With Emitter Area Adjusted For Output Power Levels, 2004 IEEE2004 IEEE MTT-S Digest”.
First of all, Japanese Patent Laid-Open No. 2003-046340 discloses a high-frequency power amplifier in which an impedance matching circuit is connected to the output sides of two power amplifying units of different saturation output and the power amplifying units are used so as to be switched at the time of high output and at the time of low output. Also, in FIGS. 9 and 10 of Japanese Patent Laid-Open No. 2003-046340, there is shown a high-frequency power amplifier in which two power amplifying units are each connected to an input matching circuit which is composed of an inductor.
Also, Japanese Patent Laid-Open No. 2004-134823 discloses a high-frequency power amplifier in which a plurality of bipolar transistors of different emitter sizes are used in the amplifier, a bias supplying means for separately controlling supply of a bias to the respective transistors is provided, and ON-OFF control is carried out depending upon the magnitude of output power.
Moreover, the above document “School of Engineering Information and Communications University (ICU)” discloses a high-frequency amplifier in which the operation of a part of amplifiers is stopped by controlling a bias supply to a semiconductor element forming a plurality of the amplifiers, whereby efficiency at the time of low output is improved.